According to Poiseuille's law (also referred to as the Hagen-Poiseuille law), the volume of a homogeneous fluid passing per unit time through a capillary tube is directly proportional to the pressure difference between its ends and to the fourth power of its internal radius (or diameter), and inversely proportional to its length and to the viscosity of the fluid.
Mathematically, Poiseuille's law is represented by:
      Δ    ⁢                  ⁢    P    =            128      ⁢                          ⁢      μ      ⁢                          ⁢      LQ              π      ⁢                          ⁢              d        4            which resolves to:
  Q  =            Δ      ⁢                          ⁢      P      ⁢                          ⁢      π      ⁢                          ⁢              d        4                    128      ⁢      μ      ⁢                          ⁢      L      where:
Q is the volumetric flow rate
ΔP is the pressure drop
π is approximately equal to 3.141592654
d is the tube diameter
μ is the dynamic viscosity of the fluid
L is the tube length.
Typically, only one pressure transducer has been employed at a first end of the tube. Atmospheric pressure is assumed to be present at the tube exit. However, in such a case, the pressure differential may not be well-controlled and may lead to inaccurate measurements of volumetric flow rate.